Botany Department, University of Toronto, Toronto, Ontario MS5 1A1, Canada

Plant-pathogen interactions are ultimately determined by molecular events that take place at the level of the cell. As our knowledge of the cell biology of eukaryotic cells has increased in recent years, so has our awareness of the extraordinary complexity that underlies even apparently simple plant responses to potential pathogens. This complexity is compounded by the spatial organization of cellular processes as well as spatial features of pathogen activities and plant responses. Elucidating the production, reception and transduction of pathogen signals, as well as plant response initiation and execution, requires not only the identification of the molecules involved but also their cellular location and movement within and between cells. The best studied plant-pathogen interactions are those that result in plant responses assumed to be defensive in nature. Although differing in detail between different types of pathogens, there are underlying similarities in these responses which result in changes in secretion, deposition of materials in the cell wall, and other examples of directed molecular trafficking. In addition, resistant plants commonly exhibit the hypersensitive response (HR) , a putative form of programmed cell death in which cellular components are destroyed in a controlled manner. Current data show considerable diversity between plant pathosystems in temporal features of this cellular destruction, and any underlying commonality in the HR still awaits elucidation. Nevertheless, its ubiquity in response to pathogen penetration of resistant cells suggests that it may be the default state in such situations, and that intracellular biotrophic pathogens have evolved ways to inhibit the response. Living, susceptible plant cells containing such pathogens often show considerable changes in cellular activities and organization that suggest remarkable interference with, and control over, cell functions by the pathogen.